Scattered light from nearby bright stars was a serious issue on
CFH12K despite all the efforts to improve the internal baffling
of the original CFHT prime focus and wide-field corrector. MegaPrime
has been designed taking note of that important constraint and the results
are very convincing: nearby off-field bright stars are not an issue anymore.

To run this crucial test, we have placed Sirius at a variety of
positions close to the mosaic to test the ability of MegaPrime
to reject scattered light from nearby bright source. Sirius was
placed a several positions North and South of the mosaic center,
ranging from 0.6 degrees to 4.0 degrees (see table) from the
center of the mosaic. All exposures
were 30 seconds with the r' filter. In 30 seconds, Sirius should
deposit roughly 4.5 x 10^12 electrons in a circle 2
arcsec diameter. The table shows the measured median sky background
on the various images. Only the 0.6 degree offset images (in any
direction), in which Sirius is nearly on the mosaic, show any
significant gradient.

Position

Median (ADU / pixel)

Image ID

North 0.6

140.689

675867o.fits

North 1.0

126.432

675868o.fits

North 2.0

122.592

675869o.fits

North 4.0

124.552

675870o.fits

South 0.6

235.002

675871o.fits

South 1.0

164.96

675872o.fits

South 2.0

147.934

675873o.fits

South 4.0

131.337

675874o.fits

East 0.6

322.102

675875o.fits

The example image from Figure 1 shows the result for Sirius placed 0.6 degrees East
of the mosaic (0.1 degree off the edge of the mosaic). The blue contour represents 600 counts per pixel and
the red represents 1200 counts per pixel. This image is scaled with
[white = 50 counts per pixel] and [black = 1300 counts per pixel]. The
plot of Figure 2 shows a cut across this image in the X direction giving counts
per second per pixel versus the approximate pixel distance from Sirius.

Figure 1. Sirius 0.1 degree off the edge, to the East.

Figure 2. Cut along the X axis from the Figure 1 image.

The following inverted images are scaled with [white = 50 counts per pixel] and [black = 500 counts per pixel].
Note that the low-level squarish structures near the center are the result of internal LED light injection
in the camera beam as these frames were taken during the first engineering phases.